The present invention relates to an improved discharge lamp which radiates ultraviolet energy of a short wavelength range, and an improved ultraviolet ray irradiation apparatus using such a discharge lamp. The present invention is suitable for use in the fields of decomposition, sterilization, disinfection, etc. of organic substances.
Ultraviolet rays of short wavelengths less than 220 nm are being used today in various fields and various applications, such as decomposition of harmful substances and organic substances. FIG. 6 shows an example of a conventionally-known closed-type liquid treating or processing ultraviolet ray irradiation apparatus, where at least one discharge lamp 30 enclosed in an outer tube (protective tube) 20 is accommodated in a cylinder 1 made of stainless steel. Liquid to be treated or processed is introduced into the cylinder 1, so that ultraviolet rays are irradiated from the discharge lamp 30 onto the introduced liquid to be processed. For example, the discharge lamp 30 used here is a low-pressure mercury vapor discharge lamp that radiates ultraviolet rays of a 185 nm wavelength (hereinafter also referred to as xe2x80x9c185 nm-wavelength ultraviolet raysxe2x80x9d). Light-emitting bulb 10 of the discharge lamp 30 is made of quartz glass having good transmissivity with respect to ultraviolet rays. The discharge lamp 30 is enclosed in the outer tube (protective tube) 20 transmissive to ultraviolet rays, so as to be isolated from the to-be-processed liquid in a liquid-tight manner. The outer tube 20 too is made of quartz glass having good transmissivity with respect to ultraviolet rays. The cylinder 1 is closed at its opposite ends with flanges 1a and 1b, and the to-be-processed liquid introduced via a liquid inlet 1c is subjected to irradiation of ultraviolet rays as it passes through the cylinder 1, and then discharged through a liquid outlet 1d. A plurality of (five in the illustrated example) reflux or baffle plates 1e-1i are provided within the cylinder 1 between the liquid inlet 1c and the liquid outlet 1d, so as to prevent a short pass of the to-be-processed liquid flowing from the liquid inlet 1c toward the liquid outlet 1d. Note that although the liquid processing ultraviolet ray irradiation apparatus is shown in FIG. 6 as including only one discharge lamp 30 for convenience of illustration, a large-capacity apparatus equipped with a plurality of the discharge lamps 30 is often used in actual cases. Ultraviolet rays emitted from the discharge lamp 30 pass through the outer tube 20 to be irradiated onto the to-be-processed liquid. For example, the irradiated ultraviolet rays function to decompose any organic substances present in the liquid into harmless carbon monoxide (CO), carbon dioxide (CO2) and water (H2O) as represented by the following mathematical expressions:
H2O+hxcexd(185 nm)xe2x86x92H+OH radical 
Cn Hm Ok+OH radicalxe2x86x92CO, CO2 and H2O 
where n, m and k represent 1, 2, 3, . . . .
The low-pressure mercury vapor discharge lamp, known as a source for emitting short-wavelength ultraviolet rays, has hitherto been applied primarily as a fluorescent lamp or sterilizing lamp. The fluorescent lamp is a discharge lamp that converts ultraviolet rays of 254 nm wavelength into visible light by means of a fluorescent substance, and the sterilizing lamp is a discharge lamp that utilizes ultraviolet rays of 254 nm wavelength. While a variety of in-depth studies have been made of the irradiation of the 254 nm-wavelength ultraviolet rays, it can not be said that the 185 nm-wavelength ultraviolet rays have attracted sufficient people""s attention or have been studied sufficiently up to the present time.
In view of the foregoing, it is an object of the present invention to provide an improved discharge lamp which can irradiate 185 nm-wavelength ultraviolet rays with an enhanced radiation efficiency and keep the enhanced radiation efficiency of the ultraviolet rays to thereby permit significant savings in energy and maintenance, as well as an ultraviolet ray irradiation apparatus using the improved discharge lamp and a method for using the ultraviolet ray irradiation apparatus.
In order to accomplish the above-mentioned object, the present invention provides a discharge lamp comprising a synthetic quartz glass tube having an inside diameter of 8 mm or over, and a pair of filaments provided within and at opposite ends of the glass tube with an L (cm) filament-to-filament distance, a mixture of rare gas and metal including at least mercury being sealed in an interior of the glass tube. In this discharge lamp, a lamp voltage V (V) and lamp current I (A) during illumination of the discharge lamp, filament-to-filament distance L (cm) and inside diameter D (cm) of the glass tube have relationship represented by the following mathematical expression:
(Vxe2x88x92Vf)/L=X/({square root over ( )}Dxc2x7{square root over ( )}I) and 2.6xe2x89xa6Xxe2x89xa64.2, 
where Vf is a constant factor depending on a illuminating power source and where if the discharge lamp is illuminated by a high-frequency power source of 1 kHz or over, Vf is 10, but if the discharge lamp is illuminated by a power source of 1 kHz or below, Vf is 50.
As will be later detailed in relation to embodiments of the present invention, the invention advantageously allows a low-pressure mercury vapor discharge lamp to radiate ultraviolet rays of a 185 nm wavelength with an enhanced efficiency and have longer life, by setting the various conditions as represented by the above-mentioned mathematical expression.
According to another aspect of the present invention, there is provided an ultraviolet ray irradiation apparatus using the novel discharge lamp arranged in the above-mentioned manner, which comprises a processing apparatus that irradiates ultraviolet rays, emitted by the discharge lamp, onto an object to be processed. Because the ultraviolet ray irradiation apparatus employs the discharge lamp capable of radiating the 185 nm-wavelength ultraviolet rays with an enhanced efficiency and having prolonged life, the irradiation apparatus can be an energy-saving type apparatus capable of operating at greatly reduced running costs.
According to still another aspect of the present invention, there is provided a method of using the ultraviolet ray irradiation apparatus arranged in the above-mentioned manner, which comprises: installing a plurality of the discharge lamps in the processing apparatus; deilluminating a predetermined number of the plurality of the discharge lamps installed in the processing apparatus and illuminating the remaining discharge lamps; and varying a combination of the discharge lamps to be deilluminated and illuminated, in accordance with the passage of time. By thus illuminating just a specific number of the discharge lamps, smaller than the total number of the discharge lamps installed in the processing apparatus, in a thinned-out fashion (thinned-out illumination) and varying the combination of the selectively deilluminated and illuminated discharge lamps in accordance with the passage of time, i.e. using the discharge lamps by rotation, it is possible to significantly prolong a time period over which the discharge lamps can operate without collective replacement with new onesxe2x80x94i.e. defer a time when the collective replacement becomes necessaryxe2x80x94, and thereby greatly save time and labor necessary for the replacement and maintenance of the discharge lamps, so that the ultraviolet ray irradiation apparatus is allowed to operate in good condition continuously for a longer time.
While the embodiments to be described herein represent the preferred form of the present invention, it is to be understood that various modifications will occur to those skilled in the art without departing from the spirit of the invention. The scope of the present invention is therefore to be determined solely by the appended claims.